In engines used for automobiles and the like, there has been widely known an exhaust turbocharger that rotates the turbine with energy of the exhaust gas of the engine in order to improve the output of the engine, and supplies to the engine the intake air by compressing the intake air by a centrifugal compressor directly coupled to the turbine via a rotary shaft.
The centrifugal compressor used for the exhaust turbocharger requires a wide operating range. When the flow rate of the centrifugal compressor decreases, an unstable phenomenon called surging occurs, and when the flow rate increases, choking occurs in the impeller or the diffuser, so that the flow rate range is limited.
In order to expand the operating range of a centrifugal compressor, there is a case of applying a casing treatment for providing a groove and a circulation passage in the casing. Although the operating range is enlarged by this application, substantial improvement cannot be expected.
Also, there is a case of expanding the operating range by applying a variable mechanism such as an entrance variable guide vane and a variable diffuser in the centrifugal compressor.
In the variable diffuser, the operating range can be significantly expanded by making a passage area variable by rotating and sliding the diffuser vane, as compared with the casing treatment.
However, in this case, a complicated drive mechanism is necessary, and the drive mechanism is costly. Moreover, there are problems in the reliability of a sliding part, a reduction in the performance due to a gap in the sliding part, gas leakage, and the like.
As prior art techniques of providing a circulation passage in the casing as one of techniques of expanding the operating range of the centrifugal compressor, there have been known Patent Document 1 (Japanese Unexamined Patent Publication No. 2007-127109) and Patent Document 2 (Japanese Unexamined Patent Publication No. 2004-27931).
Patent Document 1 discloses a technique of providing a recirculation passage by inclining an air flow out center line from an exit slit to the entrance air passage, at a certain angle toward the impeller, in the compressor that takes in a part of air from an entrance slit opened to the impeller outer peripheral air passage and takes out the intake air from the exit slit to the entrance air passage through the recirculation passage.
Also, Patent Document 2 discloses a technique of providing a circulation flow path for communicating an air entrance part to an impeller and a shroud part of the impeller, and providing an opening position on the shroud part of the circulation flow path, at a predetermined position along the meridian from a front edge of the blade.
Further, as a prior art technique of providing a variable vane to the diffuser part which is one of the expanding techniques of the operating range of the centrifugal compressor, there has been known Patent Document 3 (Japanese Unexamined Patent Publication No. 2010-65669). Patent Document 3 discloses a technique of providing a flow rate adjusting valve in either one of flow paths of a diffuser part obtained by dividing the flow path of the diffuser part.